Pulsed neutron measurement tools are useful in a variety of applications, such as subterranean operations. Examples of subterranean operations include processes for evaluating subterranean formations. For example, pulsed neutron porosity measurements involve bombarding a formation with high energy neutrons and monitoring the attenuation of the neutron flux by the formation at different distances from the neutron source. Pulsed neutron spectroscopy is based on the measurement of the spectrum of gamma rays emitted by the formation when it is irradiated by high energy neutrons. Different nuclei in formation materials emit characteristic gamma rays, allowing for the estimation of formation constituents and properties such as porosity and oil/gas saturation. Formation sigma measurements are based on the measurements of the rate of decay of a thermal neutron cloud in the formation that is created by the pulsed neutron source.
Such tools utilize scintillation detectors to detect neutrons and gamma rays. Scintillation detectors include a scintillation material and a photodetector. An ionization particle (e.g., a neutron or a gamma ray emitted in response to neutron irradiation of a formation) interacts with the scintillation material and part of the energy released in the interaction reaction is converted into photons which travel inside of the scintillator until they reach the optical window of the photodetector. The photodetector converts the photons into an output electrical signal.
The accuracy of the measurement of the energy created in the interaction of ionization radiation with the scintillation material of the detector depends on how much light emitted from the scintillation event reaches the optical window of the photodetector. This can be affected by factors such as absorption in the scintillation material and at interfaces between the scintillation material and reflective surfaces of a scintillation assembly or the photodetector optical window. Losses of the emitted light can reduce the precision and accuracy of scintillation detectors.